We have previously reported the identification of a novel cDNA, SM-20, whose corresponding mRNA levels are regulated by growth factors in rat aortic smooth muscle cell (SMC) culture. Affinity-purified polyclonal and monoclonal antibodies were made against a 230 amino acid region of the SM-20 putative peptide expressed in the bacterial vector, pET-3b. Western blot analyses of rat and human SMC lysates detected a single protein species of approximately 40 kd. SM-20 was not detected in concentrates of the culture medium. By immunohistochemistry, SM-20 was localized to filaments in the cytoplasm of cultured SMC. Like SM-20 mRNA, levels of SM-20 protein were increased 1-3 hours after serum stimulation of rat aortic SMC. In rat tissues, SM-20 antigen was detected in abundance in smooth, cardiac, and skeletal muscle. SM-20 was also detected in some epithelial cells of the kidney, pancreas, gastrointestinal tract, and skin, but was not found in the parenchyma of the liver, spleen, or testis. In the rat arterial wall, SM-20 antigen was restricted to the SMC of the media and, after balloon arterial injury, was found most abundantly in the neointima. In human atherosclerotic coronary arteries, SM-20 antigen predominated in the SMC of the intimal plaques and was not detected in macrophages, endothelium, or adventitial cells. Thus, SM-20 encodes a protein which serves as a novel SMC-specific marker within the vessel wall.